(1) Field of the Invention
The present invention relates to antennas, and more specifically to the elimination of the null along the axis coincident with the dipole moment of an antenna.
(2) Description of the Prior Art
Electrically small antennas possess a pattern in azimuth that has a null along an axis coincident with the dipole moment of the antenna. This null renders the antenna “blind” along that axis. If two antennas are used and the signals are combined, a null occurs along an axis between the two dipole moments. An ideal antenna system would be one capable of rendering equally good reception from all azimuth angles without a null in the antenna pattern(s).
It is possible to remove the null in the azimuthal response of a pair of identical orthogonal antennas by combining the two signals together ninety degrees out of phase. Adding the two signals in phase only causes the null in the azimuth pattern to shift to a position midway between the dipole moments of the two antennas.
In the past, it has been quite easy to introduce a 90-degree phase shift, at only a single frequency, by the use of a simple, single-pole electrical network. The disadvantage is that over a broad range of frequencies the user has to retune the circuit every time the frequency of operation changes. The challenge is to accomplish a ninety degree phase shift between two identical orthogonal antenna signals and be able to do so over a broad range of frequencies for example from 10 kHz to 200 kHz without exceeding a 3 dB pattern deformation.